The present invention relates to an anti-lock hydraulic brake system, i.e. a brake system by virtue of which the brake slip of a wheel can be adjusted to an optimal value during a braking operation, locking. More particularly, the present invention is related to an anti-lock brake system operating according to the "return-delivery" principle.
A brake system operating according to the return delivery principle is described in German published patent application 35 41 742. As soon as a decrease of the wheel braking pressure becomes necessary, the brake line is closed and a return line to a pump opened, which now delivers fluid from the wheel brake back into the master brake cylinder. The return line is closed and the brake line opened again for renewed pressure build-up so that fluid flows from the master brake cylinder back into the wheel brake under the effect of pedal force. It is a disadvantageous in such brakes that a driver sensed pedal movement occurs during operation of the anti-lock control . Therefore, suggestions have already been made to isolate the pump circuit from the brake cylinder and to insert a separating valve into the brake line. Use of a separating valve makes necessary a high-pressure accumulator which receives the fluid delivered by the pump. Such accumulators are usually very large and hence result in a voluminous brake unit.
Further, brake systems according to the so-termed "open-loop" system are known (e.g. German published patent application 35 05 410). The fluid is discharged out of the wheel brake into an open supply reservoir in the open-loop systems. The pump constantly delivers fluid out of the supply reservoir back into the brake circuit during a braking pressure control operation. A braking pressure control valve is operated by the master cylinder pressure which is isolated from the brake circuit by means of a separating valve and which determines the pump pressure. Fluid, which cannot get into the brake circuit will be supplied via the pressure control valve back into the supply reservoir. Such systems have the disadvantage that fluid in the brake circuit escapes through the separating valve to discharge when the pump fails. Failure of the brake circuit thus occurs.
The present invention therefore has as an object to provide a brake system which operates reliably, has no reaction or only very little reaction to the master cylinder during control operation, and which is of a compact design.